Python Flask-4.md

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1. What is Flask? (Theory)

Flask is a lightweight Python web framework used to build:

• Web applications

• REST APIs

• Backend services for mobile/web apps

Why Flask?

• Very simple and beginner friendly

• Minimal setup (no heavy configuration)

• Flexible – you choose the tools you need

• Widely used for APIs and microservices

Flask is a Micro Framework

This means:

• It provides only core features

  • URL routing

  • Request/response handling

• Other features (database, authentication, etc.) are added using extensions

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2. Flask vs Django (Quick Comparison)

Flask	Django
Lightweight	Full-stack framework
Easy to learn	Steeper learning curve
Best for APIs & small apps	Best for large applications
More control	More built-in features

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3. How Flask Works (High-Level)

1. Client (Browser / Postman) sends a request

2. Flask matches the URL route

3. Python function (view) runs

4. Flask returns a response (HTML / JSON)

Client → URL → Flask Route → Python Function → Response

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4. Installing Flask

Step 1: Check Python

python --version

Step 2: (Optional but Recommended) Create Virtual Environment

python -m venv venv

Activate:

• Windows

venv\Scripts\activate

• Mac/Linux

source venv/bin/activate

Step 3: Install Flask

pip install flask

Verify Installation

python -c "import flask; print(flask.__version__)"

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5. First Flask Application (Hello World)

Create app.py

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return "Hello, Flask!"

if __name__ == '__main__':
    app.run(debug=True)

Run the App

python app.py

Open browser:

http://127.0.0.1:5000/

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6. Understanding the Code

from flask import Flask

Imports Flask class

app = Flask(__name__)

Creates Flask application instance

@app.route('/')

URL route (endpoint)

def home():
    return "Hello, Flask!"

Function that runs when URL is accessed

app.run(debug=True)

Runs development server with auto-reload and error details

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7. What is a REST API?

REST API allows applications to communicate using HTTP methods.

Common HTTP Methods

Method	Purpose
GET	Retrieve data
POST	Send data
PUT	Update data
DELETE	Remove data

REST API returns:

• JSON (mostly)

• Not HTML pages

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8. Flask REST API – Basic Example

Import Required Modules

from flask import Flask, jsonify, request

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9. Simple GET API

from flask import Flask, jsonify

app = Flask(__name__)

@app.route('/api/hello', methods=['GET'])
def hello_api():
    return jsonify({
        "message": "Hello, this is a GET API",
        "status": "success"
    })

if __name__ == '__main__':
    app.run(debug=True)

Test URL

http://127.0.0.1:5000/api/hello

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10. GET API with Parameters

@app.route('/api/greet/<name>', methods=['GET'])
def greet(name):
    return jsonify({
        "greeting": f"Hello {name}"
    })

Example:

/api/greet/John

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11. POST API Example

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/api/add', methods=['POST'])
def add_numbers():
    data = request.get_json()
    a = data.get('a')
    b = data.get('b')

    return jsonify({
        "sum": a + b
    })

if __name__ == '__main__':
    app.run(debug=True)

Sample JSON Input (Postman / REST Client)

{
  "a": 10,
  "b": 20
}

Using Req Params

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/add', methods=['GET'])
def add_numbers():
    a = request.args.get('a')
    b = request.args.get('b')

    return jsonify({
        "a": a,
        "b": b
    })

if __name__ == '__main__':
    app.run(debug=True)

http://127.0.0.1:5000/add?a=10&b=20

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12. Understanding request and jsonify

request

• Reads data sent by client

• Used for:

  • JSON

  • Form data

  • Query parameters

jsonify

• Converts Python dictionary → JSON

• Automatically sets response headers

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13. Simple In-Memory REST API (CRUD Style)

from flask import Flask, jsonify, request

app = Flask(__name__)

students = []

@app.route('/students', methods=['POST'])
def add_student():
    data = request.get_json()
    students.append(data)
    return jsonify({"message": "Student added"})

@app.route('/students', methods=['GET'])
def get_students():
    return jsonify(students)

if __name__ == '__main__':
    app.run(debug=True)

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14. Key Flask Concepts for Beginners

Concept	Meaning
Route	URL mapping
View Function	Function handling request
Request	Data from client
Response	Data sent back
JSON	Data format for APIs
Debug Mode	Helps during development

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15. When to Use Flask?

Use Flask when:

• Teaching web development

• Building REST APIs

• Creating microservices

• Prototyping quickly

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